1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image used for development of electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing, and the like, and a binder usable for the above toner.
2. Discussion of the Related Art
As disclosed in U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809 and other publications, conventional electrophotography which can be used for apparatus for forming fixed images, such as laser printers and dry-type electrostatic copy machines, comprises the steps of evenly charging a photoconductive insulating layer (a charging process); and subsequently exposing the layer to eliminate the charge on the exposed portion to thereby form an electrostatic latent image (an exposing process); visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
Among such processes, the fixing processes include contact heat-fixing methods, such as heat roller fixing methods, and non-contact heat-fixing methods, such as oven fixing methods. Since the contact heat-fixing methods are featured in having good thermal efficiency, the temperature required for fixing can be notably lowered as compared to the non-contact heat-fixing methods, thereby making it effective in conserving energy and miniaturizing copy machines. However, in the contact heat-fixing method, there arises such a problem so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper.
In order to inhibit this phenomenon, the surface of a heat roller have been conventionally coated with a material excellent in release properties, such as a fluororesin, or a releasing agent such as a silicone oil may be applied on the surface of a heat roller. However, the method of applying a silicone oil necessitates a fixing device which is larger-scaled and complicated, making it expensive, which in turn may undesirably bring about various problems.
Conventionally, vinyl resins as typically represented by styrene-acrylic copolymers have been used for these kinds of toners. In the cases where the vinyl resins are used, when the offset resistance is intended to be improved, the softening points and the crosslinking densities of the resins have to be inevitably made higher, thereby rather making the low-temperature fixing ability poor. On the other hand, when the low-temperature fixing ability is overly prioritized, there arise problems in the offset resistance and the blocking resistance.
As disclosed in Japanese Patent Laid-Open No. 49-65232, 50-28840 and 50-81342, methods for adding paraffin waxes, low-molecular polyolefins, and the like as offset inhibitors in the process of toner making have been known. In these references, however, there arise such problems that when the amount of the offset inhibitors added is too small, sufficient effects cannot be achieved by the addition thereof, and that when the amount is too large, the resulting developer undergoes rapid deterioration. Also, when the waxy components are added during the process of toner making and blended strongly to give a uniform dispersion, since a polymer chain of the resins are liable to be broken, uniform dispersion of the waxy components while maintaining the properties inherently owned by the resins cannot be easily performed.
On the other hand, as for the binder resins for toners, polyester resins having excellent low-temperature fixing ability have been particularly used. The polyester resins have inherently good fixing ability, and as disclosed in U.S. Pat. No. 3,590,000, the toner using it can be sufficiently fixed even by a non-contact fixing method. However, since the offset phenomenon is likely to take place, it has been difficult to use these polyester resins in the heat roller fixing method. Japanese Patent Laid-Open Nos. 50-44836, 57-37353 and 57-109875 discloses polyester resins with improved offset resistance by using polycarboxylic acids. In these methods, the polyester resins still do not have good offset resistance for practical purposes. However, even if a good offset resistance is achieved, the low-temperature fixing ability inherently owned by the polyester resins is in turn becomes poor, and the pulverizability of the resin itself and that of the toner become extremely poor.
In order to solve the above problems, the following methods for blending polyester resins having excellent fixing ability with styrene-acrylic resins have been known. For instance, examples of such methods include:
(1) Methods for blending polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open Nos. 49-6931, 54-114245, 57-70523, and 2-161464); PA0 (2) Methods for chemically binding polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open No. 56-116043); PA0 (3) Methods for copolymerizing unsaturated polyesters with vinyl monomers (see Japanese Patent Laid-Open Nos. 57-60339, 63-279265, 1-156759 and 2-5073); PA0 (4) Methods for copolymerizing polyester resins having an (meth)acryloyl group with vinyl monomers (see Japanese Patent Laid-Open No. 59-45453); PA0 (5) Methods for copolymerizing reactive polyesters with vinyl monomers in the presence of polyester resins (see Japanese Patent Laid-Open No. 2-29664); and PA0 (6) Methods for forming a block copolymer by binding polyester resins and vinyl resins with an ester bond (see Japanese Patent Laid-Open No. 2-881). PA0 (1) A binder obtained by a process comprising the steps of: PA0 (2) The binder described in item (1) above, wherein the two different polymerization reactions are respectively reactions for preparing a condensation polymerization resin and an addition polymerization resin; PA0 (3) The binder described in item (1) above, wherein the component (b) is a compound having an ethylenic unsaturated bond and at least one functional group selected from the group consisting of a hydroxyl group, a carboxyl group, an epoxy group, a primary amino group, and a secondary amino group; PA0 (4) The binder described in item (3), wherein the component (b) is represented by the general formula (I): ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3, which may be identical or different, each represents a hydrogen atom, a hydroxyl group, a halogen atom, an alkyl group, an alkoxyl group, an aryl group, or a vinyl group, wherein R.sup.1 and R.sup.2 or R.sup.2 and R.sup.3 may together form a ring, each of the alkyl group, the alkoxyl group, the aryl group, the vinyl group, or the ring being substituted or unsubstituted; A and B, which may be identical or different, each represents a single bond, an alkylene group represented by the following general formula (II), or a phenylene group represented by the following general formula (III): ##STR2## wherein R.sup.4, R.sup.5 and R.sup.6, which may be identical or different, each represents a hydrogen atom, a hydroxyl group, a halogen atom, an alkyl group, an alkoxyl group, an aryl group, or a vinyl group, wherein R.sup.4 and R.sup.5 may together form a ring, each of the alkyl group, the alkoxyl group, the aryl group, the vinyl group, or the ring being substituted or unsubstituted; R.sup.7 is a single bond or a lower alkylene group; m is an integer of 0 to 5; and n is an integer of 0 to 2, and wherein X and Y, which may be identical or different, each represents --R.sup.8, --OR.sup.9, or --COOR.sup.10, wherein R.sup.8, R.sup.9, and R.sup.10 each represents a hydrogen atom or a substituted or unsubstituted lower alkyl group; PA0 (5) The binder described in item (1), wherein said releasing agent is at least one member selected from polypropylene waxes, polyethylene waxes, polypropylene-polyethylene copolymer waxes, ester waxes, and amide waxes; PA0 (6) The binder described in item (1), wherein the releasing agent has a softening point of from 70.degree. to 160.degree. C.; PA0 (7) The binder described in item (1), wherein an amount of the releasing agent is used in an amount of from 0.1 to 1000 parts by weight, based on 100 parts by weight of a total amount of the components (a) and (b); PA0 (8) The binder described in item (1), wherein the binder has an acid value of 30 KOH mg/g or less; PA0 (9) The binder described in item (1), wherein the binder has a softening point of from 70.degree. to 160.degree. C; and PA0 (10) A toner for developing an electrostatic image comprising the binder of any one of items (1) to (9).
However, since the polyester resins have inherently poor compatibility with the styrene-acrylic resins, mere mechanical blending of the components may result in poor image quality evaluation owing to such troubles as background in the formed images depending upon the blending ratios. This is because the dispersion of the resins and the internal additives such as carbon black become poor, thereby causing uneven triboelectric charges. Also, in the cases where the polyester resins and the styrene-acrylic resins have different molecular weights, the melt viscosities of the two resins are likely to differ, thereby making it difficult to have small particle sizes of the dispersed resins constituting the dispersed domains. Therefore, when such resins are used for toner making, the dispersion of the internal additives, such as carbon blacks, in the toner are extremely poor, and thereby the resulting toner drastically lacks image-forming stability. Further, in the case where the reactive polyesters are copolymerized with the vinyl monomers, it is applicable only in a restricted compositional range in order not to allow gelation.
In view of the above problems, the present inventors have developed a developer composition, which is used as a developer having both good low-temperature fixing ability and offset resistance, using a binder resin prepared by the steps of blending in advance a starting material monomer mixture of two different polymerization reaction, each of the polymerization reactions being carried out in an independent reaction path in one reaction vessel, and concurrently carrying out the two different polymerization reactions (Japanese Patent Laid-Open No. 4-142301, corresponding to U.S. Pat. No. 5,391,695).
Also, the present inventors have developed a method for producing a binder resin, further comprising the step of adding a compound which can react with both of the starting material monomers of the two different polymerization reactions in the above polymerization reaction, to thereby improve the dispersibility of the resins (Japanese Patent Laid-Open No. 7-98517, corresponding to U.S. Pat. No. 5,483,016).
However, even if resins comprising the polyester resins and the styrene resins suitably dispersed in the polyester resins were prepared by the above methods, the following limitations are posed owing to the fact that the vinyl resin constituting the dispersed domains has a narrow molecular weight distribution. In the case where the molecular weight of the vinyl resin is made low, only the fixing ability is improved but the improvement of the offset resistance is a limited. On the other hand, in the case where the molecular weight is made high, only the offset resistance is improved, but the improvement in lowering the fixing temperature is limited.
In addition, a method for producing a binder resin comprising a blend of two kinds of resins having different softening points has been developed (Japanese Patent Laid-Open No. 4-362956). Even in this method, as the blending proportion of the resin having a lower softening point is increased, the fixing ability becomes good, though the blocking resistance is undesirably small. On the other hand, as the glass transition temperature of the resin having a lower softening point is made higher, the problems in the blocking resistance are eliminated, though the improvement in the fixing ability becomes undesirably small even when the proportion added is increased. Therefore, in view of the recent demands for high speed, miniaturization, and conserved energy, further improvements in the low-temperature fixing ability and the offset resistance are desired.
An object of the present invention is to provide a binder for producing a toner having particularly good low-temperature fixing ability and offset resistance.
Another object of the present invention is to provide a toner for developing an electrostatic image comprising the binder.
These and other objects of the present invention will be apparent from the following description.